In order to meet the soaring demand for cellular telephony, including personal communication services (PCS), the capacity of these systems must be increased. The ability to transport analog signals from remote antennas to a centralized base station over fiber or fiber/coax cable would enable systems to share base station resources among many small cells. Cell size could then be reduced in a more cost-effective manner, which would lead to traffic capacity improvements.
Previous attempts have led to the development of a transmission system carrying signals to and from the base station of a wireless communications system to a remote antenna unit, wherein the transmission system could be a cable television (CATV) system, an optical link, or a wireless link. Virtually no signal processing is performed in such a remote antenna unit. Some filtering and amplification is all that is done before the signal is transmitted to the base station. Such an optical transmission system uses high performance transmitters, such as YAG lasers followed by external modulators, or high-performance distributed feedback (DFB) lasers. These type of systems are discussed in the article entitled "Fiber Optic Microcellular Radio" by Chu et al., IEEE Transactions On Vehicular Technology, Vol. 40, No. 3, pp. 599-606, August 1991; and in the article entitled "Optical Fiber-Based Microcellular Systems: An Overview" by Way, IEICE Trans. Commun., Vol. E76-B, No.9, pp. 1091-1102, September 1993.
The code division multiple access (CDMA) wireless standard, IS-95, uses spread spectrum techniques to share the available spectrum among many users. In the CDMA system multiple users transmit in the same RF channel (the same frequency band) simultaneously. This is done using a spread-spectrum technique in which each user's signal is modulated with a unique pseudo-random binary sequence (PRBS), spreading the 9.6 kbps signal over a 1.25 MHz radio frequency (RF) channel. In the IS-95 standard, the RF channel uses quadrature-phase-shift-keying (QPSK). Upon reception a correlator is used to separate the signals from multiple users; the correlator despreads only the desired signal. In an IS-95 CDMA system the uplink is fundamentally different from the downlink. The uplink is the RF channel from the user's mobile handset to the base station's antenna, the downlink is from the base station antenna to the mobile handset.
On the uplink, the RF channel is made of simultaneous voice channels originating from the users randomly distributed throughout a cell. Due to synchronization difficulties, incoherent demodulation must be used, and the codes used to spread the signals are not orthogonal. Because the codes from different users are not orthogonal, after despreading the signals from other users appear as noise, and therefore, power control is necessary on the uplink. Without stringent power control to ensure that the signal strength at the receiver is the same from all users, the weaker signals would be impaired by interference from other users within the same cell (i.e., the near-far problem).
In the downlink the entire RF channel originates from the same point, so all the signals can be synchronous. Therefore orthogonal coding can be used in spreading the signals, and the user's handset receiver can use coherent detection of the RF signal. In order to enable users to gain access to the RF channel, the downlink must also transmit control signals. From the control signals the mobile user's handset can derive the code of the access channel and synchronize its signal with that of the base station. Since the uplink and downlink use different radio link techniques, the requirements for backhauling the signals (i.e., transmitting the signals between the remote antenna unit and the centralized base station) will also differ.
The use of inexpensive Fabry-Perot (FP) lasers with CDMA signals is discussed in the article entitled "Digital Cellular In The Fiber-Optic Access Network" by O'Byrne, Optical Fiber Conference '96 Technical Digest, TuN5, pp. 83-84, February 1992, which speculates that a carrier-to-noise ratio ("CNR") of -15 dB would be sufficient, but actual implementations based on this assumption are unknown.